An analysis of the cell phone circuitry in two package bombs intercepted last week suggested the terrorists intended to delay any explosion until the US-bound planes carrying them were close to landing, US officials said Wednesday.

Authorities intercepted the two packages, sent from Yemen, in the UK and Dubai after the US was tipped off by Saudi intelligence that they were being shipped aboard UPS and FedEx flights. The packages carried addresses that previously belonged to Jewish synagogues in Chicago but were long out of date.

Officials said they have strong evidence Yemen-based al Qaeda in the Arabian Peninsula (AQAP) was behind the plot. The group’s chief bomb maker Ibrahim Hassan al Asiri is suspected in other recent attempts using innovative methods of hiding explosives.

Officials cited design changes made to maximize battery life and the outdated addresses as signs that the terrorists intended to blow up the planes in the air and delay blasts until near the end of their journeys.

Authorities said they believe al Qaeda intended to use the alarm function on the deconstructed cell phones to trigger the explosions. Officials said AQAP appeared to carry out at least one test run with harmless household items in September, possibly to time the journey to Chicago using internet tracking to monitor the shipments.

Inside the two intercepted packages were large quantities of hard-to-detect PETN explosives connected to sophisticated cell phone circuitry. The explosives were packed into printer cartridges to avoid detection.

Battery-draining features, including the screen face, were removed from the cell phones, authorities said. Because of those adjustments, the batteries in the packages might have been able to last three to four days.

Authorities said it remains unclear whether the devices would have worked as designed had they not been intercepted. AQAP attempted to blow up a US-bound passenger plane last Christmas Day, but the PETN explosives, sewn into the would-be bomber’s underwear, failed to detonate.